The storage density of optical memory devices such as magneto-optical disks is dependent on the size of the light beam for recording/reproduction use on the memory medium. Recently, a method for reproduction of storage bits smaller than the diameter of the light beam has been disclosed in Unexamined Japanese Patent Publication No. 6-150418/1994.
A magneto-optical memory medium disclosed in the above publication has a layer of a rare earth metal-transition metal alloy, and basically is composed of a first magnetic layer having characteristics whereby at room temperature it shows in-plane magnetization, while above a certain temperature a transition from in-plane to perpendicular magnetization occurs, and a second magnetic layer having characteristics whereby it shows perpendicular magnetization within a range from room temperature up to its Curie temperature.
With the above structure, when the light beam is projected onto the first magnetic layer during reproduction, the temperature distribution in the area the light beam is projected onto is a Gaussian distribution. As a result, the temperature rises only in an area smaller than the diameter of the light beam (hereinafter "temperature rise area").
This rise in temperature is accompanied, within the temperature rise area, by a shift of the magnetization of the first magnetic layer from in-plane to perpendicular magnetization. At this time, because of exchange coupling between the first and second magnetic layers, the direction of magnetization of the second magnetic layer is copied to the first magnetic layer.
In this way, information stored in the second magnetic layer is copied to the first magnetic layer in accordance with the shift of the magnetization of the temperature rise area of the first magnetic layer, and when the temperature rise area of the first magnetic layer shifts from in-plane to perpendicular magnetization, only the temperature rise area shows the magneto-optical effect. Accordingly, based on the reflected light from the temperature rise area, the information stored in the second magnetic layer is reproduced through the first magnetic layer.
Then, when the light beam moves to reproduce the next storage bit, the area of the storage bit previously reproduced drops in temperature, and shifts from perpendicular to in-plane magnetization. Because of this shift, the area where the temperature has dropped no longer shows the magneto-optical effect, and the magnetization stored in the second magnetic layer, being masked by the in-plane magnetization of the first magnetic layer, will not be reproduced. By this means, contamination of the signal from neighboring bits, which is a source of noise, can be prevented.
As discussed above, since only a temperature rise area having a temperature above a certain level is involved in reproduction, storage bits smaller than the diameter of the light beam can be reproduced, markedly improving storage density.
However, although the foregoing structure enables reproduction of storage bits smaller than the light beam, markedly improving storage density, there is the problem that the reproduction power margin becomes comparatively narrower. Specifically, since setting the reproduction power to a high level increases the area of the temperature rise area, the advantage of increased storage density is lost. Thus the reproduction power cannot be set too high. With a narrow power margin, fluctuations in the laser power can result in inability to record and reproduce information, making design of the recording and reproduction device exceedingly difficult.
A further problem with the foregoing conventional structure is that repeated laser projection tends to lead to deterioration of the characteristics of the first magnetic layer.